Abstract. It was discovered in 1995 that, during the spring time, unexpectedly low
concentrations of gaseous elemental mercury (GEM) occurred in the Arctic
air. This was surprising for a pollutant known to have a long residence time
in the atmosphere; however conditions appeared to exist in the Arctic that
promoted this depletion of mercury (Hg). This phenomenon is termed
atmospheric mercury depletion events (AMDEs) and its discovery has
revolutionized our understanding of the cycling of Hg in Polar Regions while
stimulating a significant amount of research to understand its impact to
this fragile ecosystem. Shortly after the discovery was made in Canada,
AMDEs were confirmed to occur throughout the Arctic, sub-Artic and Antarctic
coasts. It is now known that, through a series of photochemically initiated
reactions involving halogens, GEM is converted to a more reactive species
and is subsequently associated to particles in the air and/or deposited to
the polar environment. AMDEs are a means by which Hg is transferred from the
atmosphere to the environment that was previously unknown. In this article
we review Hg research taken place in Polar Regions pertaining to AMDEs, the
methods used to collect Hg in different environmental media, research
results of the current understanding of AMDEs from field, laboratory and
modeling work, how Hg cycles around the environment after AMDEs, gaps in our
current knowledge and the future impacts that AMDEs may have on polar
environments. The research presented has shown that while considerable
improvements in methodology to measure Hg have been made but the main
limitation remains knowing the speciation of Hg in the various media. The
processes that drive AMDEs and how they occur are discussed. As well, the
role that the snow pack and the sea ice play in the cycling of Hg is
presented. It has been found that deposition of Hg from AMDEs occurs at
marine coasts and not far inland and that a fraction of the deposited Hg
does not remain in the same form in the snow. Kinetic studies undertaken
have demonstrated that bromine is the major oxidant depleting Hg in the
atmosphere. Modeling results demonstrate that there is a significant
deposition of Hg to Polar Regions as a result of AMDEs. Models have also
shown that Hg is readily transported to the Arctic from source regions, at
times during springtime when this environment is actively transforming Hg
from the atmosphere to the snow and ice surfaces. The presence of
significant amounts of methyl Hg in snow in the Arctic surrounding AMDEs is
important because this species is the link between the environment and
impacts to wildlife and humans. Further, much work on methylation and
demethylation processes has occurred but these processes are not yet fully
understood. Recent changes in the climate and sea ice cover in Polar Regions
are likely to have strong effects on the cycling of Hg in this environment;
however more research is needed to understand Hg processes in order to
formulate meaningful predictions of these changes.